This invention relates generally to ethylene polymer blends. The ethylene polymer blends will have superior processability compared to generally linear polyethylenes, while providing improved physical properties over prior blends.
Since the wide acceptance of linear ethylene polymers, especially lower-density ethylene inter-polymers with incorporated linear or branched olefin comonomers, commonly called, Linear Low density Polyethylenes (LLDPE) or Very Low Density Polyethylene (VLDPE), their physical property attributes, such as toughness, have been recognized extensively.
However, for both Ziegler-Natta (Z-N LLDPE or Z-N VLDPE) and metallocene catalyst (mLLDPE or mVLDPE) catalyzed ethylene inter-polymers, their melt processing, most notably their film extrusion, is somewhat difficult, especially as compared to products which came before, such as branched High Pressure produced Low-Density Polyethylenes (HP-LDPE). This difficulty has led manufacturers of melt extruded films or molded articles to blend the more easily processed ethylene polymers, usually highly long-chain branched HP-LDPE, into the LLDPEs.
However, as with many such compromises, gaining processability with the addition of amounts of HP-LDPE generally leads to a lowering of physical properties of such blends, when compared to the physical properties of either mLLDPE or Z-N LLDPE materials by themselves.
There is a commercial need therefore for an ethylene inter-polymer composition that can equal or exceed the processability of HP-LDPE blends while offering improved physical and optical properties over the HP-LDPE/LLDPE blends.
We have discovered that when a metallocene catalyst-produced first ethylene inter-polymer with narrow to medium molecular weight distribution (MWD), also known as polydispersity index or PDI, a narrow comonomer composition and controlled levels of long chain branching (LCB), is blended with a second ethylene inter-polymer (e.g. one or more of Z-N LLDPE, mLLDPE, Z-N VLDPE, mVLDPE), the resulting ethylene inter-polymer blend composition has lower melt viscosity, improved optical propertial and improved physical properties, when compared to HP-LDPE blends with LLDPE.
These blends deliver the desired processability, equaling or exceeding the processability of HP-LDPE/LLDPE blends.
In other embodiments, an ethylene inter-polymer blend composition is contemplated, comprising greater than 1 weight percent of at least one first ethylene inter-polymer, the first ethylene inter-polymer, having a branching factor of greater than 0.4 or less than 0.7, a polydipersity index, (PDI) ratio of weight average molecular weight to number average molecular weight) greater than 3 or less than 15, and a normalized relaxation spectrum index (nRSI), as definded below greater than 26 and a Crystallizable Chain Length Distribution Index (CCLDI), the CCLDI as described in U.S. Pat. No. 5,789,427 (""427) incorporated herein by reference, of greater than 1.0 or less than 10.0.
Additionally, the ethylene interpolymer blend is comprised of less than 99 weight percent of at least one second ethylene inter-polymer, the second ethylene inter-polymer having a branching factor greater than 0.7, a PDI greater than 1.5 or less than 6, normalized relaxation spectrum index nRSI greater than 1 or less than 11, and a CCLDI greater than 1 or less than 20.
In further embodiments, a film made from the ethylene inter-polymer blends is contemplated to have superior optics and superior toughness. The film may have a dart drop impact greater than 100 g/mil, the film may have a haze less than or equal to 7% any may have a Elmendorf tear in the machine direction (MD) greater than 80 g/mil. All three of these properties and values may be found in the films simultaneously.
The polymer blend compositions described in this invention can be produced by post reactor blending, of two or more components, produced in staged reactors, in a single reactor via mixed catalysts, or combinations of these techniques.